This invention relates to boron doping of semiconductors, and more specifically to the achievement of a high degree of uniformity and reproducibility in the boron doping of many large diameter silicon semiconductor wafers at the same time.
Various dopant impurities are used to alter the electrical properties of silicon so that different device and circuit functions can be achieved. It is essential that the doping process give reproducible and uniform results, that is, reproducible from manufacturing run to manufacturing run, and uniform within a given manufacturing run so that, insofar as possible, each location on the semiconductor wafer and each semiconductor wafer receives substantially the same concentration of dopant and the same penetration depth into the wafer. The element boron has been found to have useful and convenient properties as an impurity dopant in silicon and is much used. Therefore, manufacturing methods which provide boron doping of silicon to a high degree of reproducibility and uniformity are of great technical and practical utility.
A useful technique for impurity doping of silicon is the open-tube system wherein a batch of wafers is inserted into a furnace tube or reaction chamber placed within a high temperature furnace having a temperature near 1000.degree. C. A controlled atmosphere is maintained within the tube. The tube is open in the sense that various gases can be introduced, flow through the tube or reaction chamber, and be extracted. A gaseous species containing boron is either introduced directly into the tube, or generated within the tube by reaction or evaporation of a boron compound. The gas stream may also contain relatively inert carrier gases such as N.sub.2, Ar or He, and/or active species such as O.sub.2, H.sub.2 or H.sub.2 O, for example, capable of oxidizing or reducing the boron dopant source compound and/or the silicon material within the diffusion tube. Alternatively the inert carrier gas may be omitted and the active gases used without dilution at various pressures. BCl.sub.3 is a typical boron source gas and is used to illustrate the reactions which can occur.
Reaction of BCl.sub.3 in a reducing atmosphere produces elemental boron as a doping impurity source for the silicon, and HCl which reacts with Si to form SiCl.sub.4. The SiCl.sub.4 is gaseous at the temperatures of interest, so the reaction also etches the silicon, an effect generally desired to be avoided. In oxidizing atmospheres, BCl.sub.3 reacts to produce B.sub.2 O.sub.3 and SiO.sub.2 as reaction products, which deposit or form on the silicon wafers. HCl is also produced and can etch or pit the silicon through formation of SiCl.sub.4, but this reaction is less likely than the formation of SiO.sub.2, so that the problem is less severe than in reducing atmospheres. Hence, oxidizing ambients have generally been preferred in the prior art, and a large excess of oxidant typically used, for example, ten times or more of the amount needed for stoichiometric productions of B.sub.2 O.sub.3.
The boron oxide deposited on the silicon surface provides the primary source of boron dopant atoms rather than the gas stream itself. This permits the doping process to be divided into two sequential steps which, for reasons of control, are carried out at slightly different temperatures; (a) "deposition" typically carried out in the range 700.degree.-900.degree. C. in which a B.sub.2 O.sub.3 +SiO.sub.2 mixture is formed on the silicon wafers; and (b) "drive" in which the boron source gas is removed and the silicon wafers raised to a higher temperature (typically 900.degree.-1100.degree. C.) in order to accelerate solid-solid diffusion of the boron from a B.sub.2 O.sub.3 /SiO.sub.2 glassy layer into the silicon semiconductor body. The deposition step provides primary control of the surface impurity concentration, and the drive step provides primary control of the depth to which diffusion is achieved. The above-described reactions are well known per se in the art.
In the open tube method, the boron source gas and other reactant gases (hereinafter the composite gas) pass continuously through the tube. As the composite gas stream passes over the silicon wafers and the deposition- oxidization reactions occur, there is depletion of the active species from within the gas stream. The thickness of the dopant source oxide formed on the first wafers (nearer the gas source end) may be many times that formed on the last wafers in the batch, nearer the exhaust or pump end. There will also be depletion of the reactants from the periphery to the center of the wafers. These depletion effects give rise to non-uniformity in the formation of the boron dopant source oxide layer and consequently on the resultant impurity concentration and distribution within the silicon.
The smaller the spacing between wafers and the larger the diameter, the more severe the depletion. Thus in the prior art, wafers could not be closely spaced within the reaction chamber in order to process large numbers in a single run without suffering significant depletion and non-uniform deposition effects. Conversely, if greater uniformity was desired, larger wafer spacings were mandatory, and production efficiency suffered. As wafer diameter has increased from two inches (5.1 cm) to four inches (10.2 cm) or larger, these spacing and uniformity problems have become especially troublesome.
It is also known that B.sub.2 O.sub.3 can dissociate in the presence of silicon at high temperatures (&gt;800.degree. C.) to form what is believed to be an intermetallic boron-silicon compound SiB.sub.X that is soluble only in solvents which attack silicon. The SiB.sub.X can be advantageously used to enhance doping uniformity, but its undesirable etch properties must be avoided in some way so that subsequent process steps may be accomplished without etching the silicon.
In the prior art, a variety of approaches have been utilized to achieve uniform doping by the minimization of the reactant depletion effects. For example, (a) use of high composite gas flow rate (e.g. 0.1-10 liters per minute), (b) use of large wafer spacing (e.g. 0.5-10 cm), which limits the number of wafers that can be treated in a single batch, and (c) use of low pressures (e.g. &lt;1 Torr, 0.13 kPa) so as to increase the mean free path of gas molecules within the reaction chamber and enhance surface reaction. However, none of these approaches has proved entirely satisfactory. A need continues to exist for a process capable of producing uniform and reproducible boron doping of silicon which is both economical with respect to gas usage, able to handle batches of a hundred or more silicon wafers of four inch (10.2 cm) diameter or larger in a single uninterrupted diffusion run, and which is also compatible with other desired process steps.
Boron is a p-type impurity in silicon and, when diffused into an n-type silicon body, forms a p-n junction. In this circumstance, a convenient method of evaluating the results of the diffusion operation is to measure the sheet resistance R.sub.s in ohms per square and the junction depth X.sub.j in microns. The methods for performing these measurements are well known per se in the art.
In view of the foregoing, it is an object of this invention to provide an improved method for uniform and reproducible boron doping of silicon.
It is a further object of this invention to provide an improved method for uniform and reproducible boron doping of silicon at low pressure.
Another object of this invention is to provide for uniform and reproducible boron doping of silicon with smaller reactant gas consumption.
Another object of this invention is to provide an improved method for uniform and reproducible boron doping of silicon while preventing undesirable etching of the silicon surface.
Another object of this invention is to provide an improved method for controlled oxidization and boron doping of silicon.
Another object of this invention is to provide an improved method for the formation and subsequent removal of a boron-silicon intermediate layer at the surface of the silicon semiconductor as an aid to obtaining improved uniformity and reproducibility of boron doping in silicon without disruption of subsequent process steps.
A further object of this invention is to provide a method for uniform and reproducible boron doping of large diameter silicon wafers which are closely spaced in the reaction chamber so that relatively large numbers can be processed simultaneously in a single batch to reduce manufacturing costs.